1. Field of the Invention
The present invention relates to a semiconductor circuit device including a switching element, and particularly to a semiconductor circuit device including a direct current (DC) to direct current (DC) conversion circuit or a direct current (DC) to alternating current (AC) conversion circuit using a switching element.
2. Description of the Background Art
In DC-DC conversion circuits (DC-DC converters) and DC-AC conversion circuits (inverter circuits), overheating due to switching loss in a semiconductor switching element often becomes problematic. As a method of suppressing overheating of a semiconductor switching element, for example, the following methods are known.
A method described in Japanese Patent Laying-Open No. 03-178565 is directed to an inverter device in which a switching element in an inverter main circuit is switched in response to a pulse width modulation signal having a given carrier frequency. The inverter device described in this document includes temperature detection means detecting a temperature of the switching element, comparison means comparing the temperature detected by the temperature detection means with a reference temperature, and correction means correcting the carrier frequency of the pulse width modulation signal based on an output signal from the comparison means.
Japanese Patent Laying-Open No. 2005-198406 discloses a boost device (boost chopper) capable of reducing switching loss. In the boost device, a gate resistance value of an IGBT is optimized in accordance with a boost voltage. Thereby, the switching loss can be reduced.
Similarly, in semiconductor integrated circuits, as a result of an increase in power consumption along with an improvement in the degree of integration, overheating and thermal runaway of an integrated circuit often become problematic. As a technique for reducing power consumption, for example, Japanese Patent Laying-Open No. 2008-124125 discloses a semiconductor integrated circuit device with a configuration described below. The semiconductor integrated circuit device includes a temperature sensor capable of detecting a temperature, determining, for each reference value, whether or not the detection result exceeds the reference value, and outputting a signal representing the determination result, and a control block capable of controlling an operation of a calculation block based on the output signal of the temperature sensor. The control block returns from a suspended state to an operation state by an interrupt signal based on the output signal of the temperature sensor, and determines an operation condition for the calculation block to satisfy a temperature condition for the calculation block.
In addition to a semiconductor switching element, overheating of a motor connected to an inverter circuit may become problematic. Japanese Patent Laying-Open No. 08-182387 discloses a method for suppressing temporary overheating of an AC motor without stopping an operation of the AC motor. According to the method, if a Central Processing Unit (CPU) detects that a temperature detected by a temperature detection element attached to a stator winding of the AC motor exceeds an upper set temperature, the CPU decreases an inverter output frequency by Δf with an appropriate time gradient. If the detected temperature becomes not more than a lower set temperature, the CPU increases the inverter output frequency by Δf with an appropriate time gradient.
In a Pulse Width Modulation (PWM) DC-DC converter, a higher switching frequency is preferable because ripples in an output are reduced and a coil (reactor) can be downsized. However, as the switching frequency is increased, high frequency loss in the coil is increased, and thus overheating due to the high frequency loss becomes problematic. Similarly, in a case where a motor is driven by a PWM inverter circuit, if a carrier frequency is increased, overheating of a stator winding due to high frequency loss becomes problematic.
As a method of suppressing overheating of a coil, a stator winding, and the like, it is contemplated to reduce an output voltage of a DC-DC converter, or to reduce an output voltage (current) or an output frequency of an inverter circuit. Although such a method is effective as a method of suppressing temporary overheating, it has a problem in a case where a device connected to a DC-DC converter or an inverter circuit is steadily operated. For example, in a system in which an inverter circuit is driven by an output of a DC-DC converter and a motor is driven by an output of the inverter circuit, overheating of a coil provided to the DC-DC converter can be suppressed by reducing an output voltage of the DC-DC converter. However, since it is necessary to increase an output current of the inverter circuit to maintain an output of the motor, overheating may occur in a stator winding of the motor.